Contact cements or adhesives are polymer solutions or dispersions which are applied to two surfaces, dried, and the mating surfaces are then pressed together, usually without heat. The dried surfaces are essentially non-tacky and will not adhere to most materials except to another coating of the same adhesive. Care must be used in aligning the two articles or surfaces to be bonded, since the bonding is essentially instantaneous and the articles cannot be moved relative to one another in order to put them into register once contact has been made. This "instant-grab" property of contact adhesives has the important advantage that long periods of aging or oven curing are not needed. Particularly for on-site applications, such as laminating a plastic surface sheet material to a kitchen countertop, the advantages of the use of contact adhesives are particularly valuable.
Pressure sensitive adhesives are known which involve blends of polymers. For example U.S. Pat. No. 3,090,694 in Examples 7 and 8, disclose blends of acrylic latexes. In Example 7 the first product ("Rhoplex".RTM.FRN latex) is known to have a number average molecular weight (M.sub.n) of about 500,000 and the second product ("Rhoplex" B-60A latex) to have a molecular weight of about 1,000,000. In Example 8 the first product ("Rhoplex" B-15 latex) is understood to have a M.sub.n in the neighborhood of 750,000 and again the second product ("Rhoplex" B-60A latex) has a M.sub.n of about 1,000,000. The glass transition temperature of Rhoplex FRN is about -15.degree. C., that of Rhoplex B-60A is about 13.degree. C. and that of Rhoplex B-15 is about -12.degree. C. Additionally, U.S. Pat. Nos. 3,222,419 and 3,257,478 disclose blends of crosslinking acrylic resins and non-crosslinking acrylic resins, in the form of organic solvent solutions, used as pressure sensitive adhesives.
As is shown by U.S. Pat. Nos. 2,976,203 and 2,976,204 to Young (assigned to the assignee of the present application) it is known to utilize chain transfer agents to lower the molecular weight of acrylic latex polymers useful as pressure sensitive adhesives and as contact cements. Another reference showing that chain transfer agents are known for making pressure sensitive adhesives is U.S. Pat. No. 3,806,484, in which the acrylic monomers are pre-emulsified along with the chain transfer agent, or, as understood, the emulsion is polymerized in increments wherein the chain transfer agent is added in the same quantity to each stage or increment of the emulsion being polymerized.
Formulations for contact cements utilizing acrylic latexes and phenolic resins are discussed in the 1975 edition, Vol. 9A, of Polymer Science and Technology on pages 233-48 (abstracted at Chemical Abstracts Vol. 84 Abstract 136482a). A review of contact adhesives is mentioned in Volume 81 of Chemical Abstracts, Abstract No. 121991e. German patent publication No. 2,420,683, cited in volume 82 of Chemical Abstracts, Abstract No. 59158d, discloses a contact adhesive and refers to U.S. Ser. No. 355,676. Elastomers are known to have a wide range of molecular weight polymer chains.
A patent relating to synthetic rubber, U.S. Pat. No. 4,145,494, shows the polymerization of diolefins, optionally with minor proportions of monoethylenically unsaturated monomers such as acrylic acid, methacrylic acid, itaconic acid, esters thereof, styrene, etc., during which larger amounts of chain transfer agents are added after at least 75% of the monomers are polymerized. A shortstopping agent is added to prevent complete conversion of the monomer to polymer. No such agent is used in the present invention.
One way of carrying out a synthesis procedure to obtain a latex polymer having a wide divergence of molecular weights is taught by U.S. Pat. No. 4,039,500 to Bassett et al. It teaches, in one variation, feeding a stream of unemulsified monomer mixture containing a chain transfer agent into a second body of unemulsified monomer simultaneously with continuous feeding of a stream of the second body of monomer into a polymerization reactor which contains water, an emulsifying agent and an initiator. A polyunsaturated crosslinking monomer is preferably included. Example 1 of the patent is typical, yielding an overall polymer composition of 45/45/5/5 styrene, ethyl acrylate, methacrylic acid, 2-hydroxyethyl acrylate, which has a calculated Tg, defined hereinafter, of about 28.degree. C. The first feed would yield a polymer having a lower Tg and the second feed would yield a polymer having a higher Tg, probably in a so-called shell-core relationship.
That process has deficiencies as compared with the process of Graham Swift and Richard E. Baus, discussed below and disclosed in certain of the examples in the present case. For instance, the Bassett et al. process is difficult to control. The Swift et al. process, on the other hand, allows better control of the proportions of very high molecular weight and very low molecular weight materials. Furthermore, Bassett et al. are concerned with getting a hard polymer for paints, using substantial proportions of monomers such as styrene and methyl methacrylate, which give products having a high Tg. Bassett et al. also recommend using substantial amounts of the unsaturated crosslinker monomers, which give extremely high molecular weight products as well as a high Tg.
Contact adhesives differ from pressure sensitive adhesives in that they are essentially non-tacky (although they adhere to one another), while pressure sensitive adhesives retain a permanent, aggressive tack. This may be illustrated by comparing tack ratings of contact and pressure sensitive adhesives in conventional tack tests such as the Rolling Ball Tack Test. In this test, a 7/16" steel ball is rolled down a 6 inch, 45.degree. inclined steel sheet onto the horizontal adhesive surface in the form of a dry coating of about 1 mil in thickness. A pressure sensitive adhesive is expected to stop the ball after it has travelled only one or two inches across the adhesive surface. Conversely, contact adhesive will permit the steel balls to roll almost indefinitely.
Traditionally, contact adhesives have been based on solutions of poly(chloroprene) or neoprene, in combinations of solvents such as toluene and methyl ethyl ketone. Recent government regulations restricting use of such solvents has sparked adhesive manufacturers to seek alternate systems which pose significantly lower flammability and pollution hazards.
Aqueous adhesives seem to be ideal candidate replacements for the hazardous solvent systems. However, early aqueous neoprene latices lacked the high bond strengths shown by their solvent counterparts, nor are they stable to freezing and thawing. In a subsequent innovation, it was found that the deficiency could be overcome by using specialty acrylic latices such as 87.5 ethyl acrylate/10 methyl methacrylate/2.5 itaconic acid+poly(vinyl alcohol) thickener and benzoguanamineformaldehyde crosslinker. While this may be used without safety hazards to provide high strength wood-to-plastic laminates, the system requires higher lamination pressure than customers desire, and thus greater combinability was sought. Improved combinability, as evidenced by good adhesion with low lamination pressure, can be achieved by incorporation of tackifiers and plasticizing solvents, reducing Tg, or dropping molecular weight. However, in all cases combinability is achieved along with an unacceptable loss is elevated temperature performance.
Extensive testing of contact adhesives has indicated that performance in terms of adhesive properties such as lap shear, high temperature cleavage, legginess, combinability, etc., is related to the type and amount of acid functionality in the polymer backbone and the type and amount of crosslinker. However, it is apparent that performance is even more dramatically related to molecular weight of the polymer. High molecular weight improves lap shear adhesion and cleavage properties, but downgrades legginess, combinability, and bond fusion. The exact opposite is true of low molecular weight polymers.